Alexander Fleming accidentally discovered penicillin when he noticed that a mold called Penicillium notatum had killed the bacteria in one of his Petri dishes. This was the world's first 'antibiotic'—a substance that could kill harmful bacteria without damaging human cells. However, Fleming struggled to isolate and mass-produce the drug. It wasn't until the 1940s, during WWII, that Howard Florey and Ernst Chain developed the chemical engineering processes required to produce enough penicillin to treat soldiers. This invention revolutionized medicine, turning previously fatal infections like pneumonia, syphilis, and gangrene into treatable conditions. It is estimated that penicillin has saved over 200 million lives since its first use, radically increasing global life expectancy and forming the basis for the modern pharmaceutical industry.